Electrophotography is a useful process for printing images on a receiver (or “imaging substrate”), such as a piece or sheet of paper or another planar medium (e.g., glass, fabric, metal, or other objects) as will be described below. In this process, an electrostatic latent image is formed on a photoreceptor by uniformly charging the photoreceptor and then discharging selected areas of the uniform charge to yield an electrostatic charge pattern corresponding to the desired image (i.e., a “latent image”).
After the latent image is formed, charged toner particles are brought into the vicinity of the photoreceptor and are attracted to the latent image to develop the latent image into a toner image. Note that the toner image may not be visible to the naked eye depending on the composition of the toner particles (e.g., clear toner).
After the latent image is developed into a toner image on the photoreceptor, a suitable receiver is brought into juxtaposition with the toner image. A suitable electric field is applied to transfer the toner particles of the toner image to the receiver to form the desired print image on the receiver. The imaging process is typically repeated many times with reusable photoreceptors.
The receiver is then removed from its operative association with the photoreceptor and subjected to heat or pressure to permanently fix (i.e., “fuse”) the print image to the receiver. Plural print images (e.g., separation images of different colors) can be overlaid on the receiver before fusing to form a multi-color print image on the receiver.
Electrophotographic (EP) printers typically transport the receiver past the photoreceptor to form the print image. The direction of travel of the receiver is referred to as the slow-scan, process, or in-track direction. This is typically the vertical (Y) direction of a portrait-oriented receiver. The direction perpendicular to the slow-scan direction is referred to as the fast-scan, cross-process, or cross-track direction, and is typically the horizontal (X) direction of a portrait-oriented receiver. “Scan” does not imply that any components are moving or scanning across the receiver; the terminology is conventional in the art.
The magnitude of the charge on the toner particles is of vital importance in electrophotography and generally limits both the amount of toner deposited in an area and the size of the toner particles. This is discussed in commonly-assigned U.S. Pat. No. 8,147,948 to Tyagi et al., entitled “Printed article,” which is incorporated herein by reference. Specifically, the amount of toner deposited to convert the electrostatic latent image on the photoreceptor is proportional to the difference of potential between a development station that is used to transport the electrically charged toner particles into operative proximity to the latent image bearing photoreceptor and the photoreceptor. The photoreceptor is initially charged to a potential using known means such as a corona or roller charger and an electrostatic latent image is formed on the photoreceptor by image-wise exposing, thus discharging the photoreceptor in an image-wise fashion. The initial potential is limited by the dielectric strength of the photoreceptor. For a typical organic photoreceptor commonly used today, the initial potential is limited to less than approximately 500 V. The potential on the development station is limited by the necessity of not depositing toner particles in un-toned areas. Thus, the magnitude of the minimum difference of potential must be sufficient to preferentially attract the charge toner particles towards the development station in regions where toner particles should not be deposited on the photoreceptor.
After development of the electrostatic latent image to convert the electrostatic latent image into the toner image, the toner image is transferred from the photoreceptor to a receiver such as paper. Transfer is generally accomplished by transporting the toner image-bearing photoreceptor into contact with a receiver and subjecting the photoreceptor-receiver to an electrostatic field and pressure that urges the toner particles to transfer from the photoreceptor to the receiver. Countering the applied electrostatic forces resulting from the applied electrostatic field are electrostatic forces between the charged toner particles and the photoreceptor and surface forces such as those arising from van der Waals interactions that adhere the toner particles to the photoreceptor. The applied electrostatic force must be sufficient to overcome the forces that hold the toner to the photoreceptor in order for the toner particles to be transferred to the receiver.
The applied electrostatic force exerted on a toner particle is the product of the charge on the toner particle times the applied electrostatic transfer field. Increasing the charge on a toner particle increases the adhesion of that particle to the photoreceptor. Moreover, the field generated by the charged toner particles counters and reduces the applied electrostatic transfer field. Thus, increasing toner charge decreases the force available to transfer the toner particles from the photoreceptor to the receiver. This makes transfer more difficult. In addition, increasing toner charge also limits the amount of toner that is deposited during the development process when the electrostatic latent image is converted into a visible image. It is obvious that the amount of charge that can be imparted onto a toner particle is necessarily limited.
The magnitude of the electrostatic transfer field is limited by the Paschen discharge limit of air. Air can support a maximum applied field, known as the Paschen limit. The Paschen limit decreases with increasing air gap. For a 10 μm air gap, the limit is approximately 35 V/μm. As the size of the gap increases, as would occur when making raised letter printing or other applications that require the formation of macroscopic toner structures such as Braille, textured effects, etc. the size of the electrostatic transfer field that can be applied decreases as the size of the relief pattern generated to provide the raised lettering or macroscopic toner structures increases. Moreover, the presence of macroscopic relief structures generally requires the presence of large quantities of electrically charged toner particles. The charge on the toner particles generates an electrostatic field that subtracts from the applied field in the presence of the toner structure while the air gap in the vicinity around the relief structure limits the size of the applied field due to the Paschen discharge limit. Accordingly, it is often not possible to electrostatically transfer macroscopic toner structures generated when forming macroscopic toner structures from the photoreceptor to a receiver. It is clear that a new method of forming macroscopic toner relief patterns is necessary.
There is a need, therefore, for a method capable of developing, transferring, and fusing stacks of toner particles whereby the toner stacks are of sufficient height that allows them to be sensed using tactile means.